Diet-induced obesity, insulin (INS) resistance, and type 2 diabetes have reached epidemic proportion among women in the United States, and each of these health problems significantly increase a woman's risk of developing breast cancer. Studies linking exposure to the endocrine disruptor Bisphenol A (BPA) with mammary tumorigenesis in rodents, poor survival outcome and response to chemotherapy in human breast cancer, and an increased risk for diabetes and INS resistance are suggestive of a shared signaling network responsive to this ubiquitous environmental contaminant. BPA readily crosses the placental barrier, appears to accumulate in the fetus, and is found in breast milk However, very little is known about the molecular mechanisms that regulate breast cancer development in response to co-morbidities like obesity and INS resistance, or how BPA functions in this context. While the true molecular target of BPA has been debated for some time, it is now known that this compound can bind with high affinity and specificity to the orphan nuclear receptor estrogen-related receptor gamma (ERR?). In our preliminary studies, we have found that a neonatal exposure to BPA impairs glucose tolerance in 2-month-old female Sprague Dawley rats, suggestive of early INS resistance, and hepatic expression of ERR? and two of its putative target genes (hexokinase 2, HK2;pyruvate dehydrogenase kinase 4, PDK4) are increased by BPA exposure in these animals. HK2 and PDK4 are essential enzymes of glycolysis, dysregulation of which is implicated in diabetes, INS resistance, and cancer. BPA also increases the expression of HK2 and PDK4 in MCF10A normal human mammary epithelial cells, and transient overexpression of ERR? cDNA mimics BPA's effect on HK2, suggesting that BPA-mediated changes in vivo are likely to be ERR?-dependent. Our central hypothesis is that early life exposure to BPA increases later mammary cancer risk by reducing INS sensitivity, and that this is mechanistically dependent upon ERR?, which we will test in two Specific Aims: Aim 1 will whether early life exposure to BPA and subsequent exposure to a high-fat diet reduces INS sensitivity and increases mammary tumorigenesis in wildtype mice, and whether this is reduced or fails to occur in mice exhibiting loss of one ERR? allele. Aim 2 will study ERR?, HK2, and PDK4 expression changes in the mammary glands and tumor tissues of ERR? mice and their wildtype counterparts to determine whether the activity of this glycolytic signaling network is associated with the INS resistance and tumorigenicity measures from Aim 1. Our proposed studies of INS resistance and mammary tumorigenesis in response to early life exposure to BPA and diet-induced obesity, and their dependence on the orphan nuclear receptor ERR?, represent a novel and mechanistic approach to the investigation of gene-environment-diet interactions, and are poised to make transformative contributions to our understanding of breast cancer prevention.